An obscure chemical in hard plastic baby bottles, liners inside canned food and some water containers lies at the center of controversy as the California Legislature considers a bill to ban it in children's products.

If passed, California would be the first state to limit its use.

Bisphenol A -- the prime chemical in making the polycarbonate plastic popular in durable, clear Nalgene water bottles -- has come under increasing scrutiny in recent years from scientists who caution that it's found in thousands of consumer products and has invaded the human body.

But researchers have found that at doses below or at a federal safety guideline, the chemical can disrupt hormone systems of lab animals, affecting the workings of their brains.

Bisphenol A has been used for decades in the manufacture of tough plastics known as polycarbonate plastics. The plastics make up a wide variety of products, primarily food and drink packaging and containers such as hard, clear and sometimes tinted Nalgene water bottles, and in toys, pacifiers, baby bottles and teethers.

The chemical is also used in epoxy resins that coat food cans, bottle tops and water supply pipes, and as sealants for children's teeth for the prevention of cavities.

Assemblywoman Wilma Chan, D-Oakland, became aware of the chemical's possible hazards through her work on the Select Committee on Children's Health and School Readiness and introduced a bill last month that would prohibit the manufacture or sale of any product intended for use by a child 3 years of age or younger, if it contains bisphenol A.

The bill, AB319, also would ban in toys and child care articles certain forms of phthalates, plastic softeners.

The committee, which she heads, has turned up a number of studies showing that bisphenol A and some phthalates can cause hormone and nerve damage in young children, she said.

Chan, whose legislation led to the banning of two forms of flame retardant two years ago, said she was "shocked to find out that there were chemicals in toys that babies put in their mouths and in baby bottles."

"We just shouldn't have these products on the market in California," Chan said.

According to 1999 industry data, about 2 billion pounds of bisphenol A are produced yearly in the United States. In the last 10 years, criticism has grown with studies showing that bisphenol A can leach from products under high heat and alkaline conditions, and the rate of leaching is affected by the age, condition and wear of the products.

The American Plastics Council, which represents companies that use bisphenol A, maintains that the amount of leaching isn't significant. The group opposes the Chan bill.

"The bill is written fairly broadly," said spokesman Steve Hentges. A ban on bisphenol A could potentially eliminate the coating used to line cans to prevent metal from corroding into foods. "You can't make polycarbonate without it."

At this point, Chan said, her bill doesn't attempt to regulate the bisphenol A in food cans.

As for the health effects, Hentges said, "The evidence has been examined by governments and scientific bodies worldwide. In every case, the weight of evidence supports the conclusion that bisphenol A is not a risk to human health at the extremely low levels to which people might be exposed."

But an author of one of the new studies, Thomas Zoeller, a thyroid endocrinologist and chairman of the University of Massachusetts' biology department, said researchers had shown that humans were widely exposed to bisphenol A, a chemical that can disrupt animal hormone systems that affect the workings of the brain.

Further, it appears to accumulate at higher concentrations around the fetus -- in the umbilical cord and the amniotic fluid -- than in the mother's blood, said Zoeller, a leading authority on fetal thyroid development. While it's not clear what the affects are on humans, Zoeller and his colleagues published a study in the journal Endocrinology in February showing that, in lab animals, bisphenol A altered the ability of thyroid hormone to correctly regulate brain development.

In another study, expected in an upcoming issue of the journal Neuroscience, a University of Tokyo group found that bisphenol A inhibited the positive role of estrogen in enhancing neural connections in a part of the brain involved in the formation and retention of memory, the hippocampus.

Scientists recognize that an increased rate of synapse formation may benefit memory, and the hormone estrogen in both the female and male brain is important in terms of increasing the density of synapses.

Yet, when the Yale and Helen Hayes researchers injected extremely low doses of bisphenol A in rats, "the positive effect of the estrogen was strongly inhibited because there were fewer synapses," said Neil J. MacLusky, a developmental neuroendocrinologist at the hospital's Center for Neural Recovery and Rehabilitation Research.

"We don't know that it necessarily means anything for human beings," he said. "But if these kinds of biological effects occur in humans, it raises serious issues."